Why not stopping the crime and let the would-be-perpetrator go (assuming he didn't cross any legal lines already)? They're not arresting them for crimes they didn't commit (but were predicted); they're arresting them for futures crimes they would commit (and aren't yet predicted). If you let them go, you can arrest them again the next time they want to act, since you will predict it again.
Why not stopping the crime and let the would-be-perpetrator go (assuming he didn't cross any legal lines already)? They're not arresting them for crimes they didn't commit (but were predicted); they're arresting them for futures crimes they would commit (and aren't yet predicted). If you let them go, you can arrest them again the next time they want to act, since you will predict it again.